The requirement for increasingly high areal recording density imposes increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S.sup.*), medium noise, i.e., signal-to-noise ratio (SNR), and narrow track recording performance. Efforts to produce a magnetic recording medium satisfying such demanding requirements confront significant challenges.
The linear recording density can be increased by increasing the coercivity of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al) -alloy, such as an Al-magnesium. (AlMg) alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, ceramic and glass-ceramic materials, plastics, silicon, as well as graphite. There are typically sequentially sputter deposited on each side of substrate 10 an adhesion enhancement layer 11, 11', e.g., chromium (Cr) or a Cr alloy, a seedlayer 12, 12', such as NiP, an underlayer 13, 13', such as Cr or a Cr alloy, a magnetic layer 14, 14', such as a cobalt (Co)-based alloy, and a protective overcoat 15, 15', such as a carbon-containing overcoat. Typically, although not shown for illustrative convenience, a lubricant topcoat is applied on the protective overcoat 15, 15'.
It is recognized that the magnetic properties, such as Hr, Mr, S.sup.* and SNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is recognized that underlayers having a fine grain structure are highly desirable, particular for epitaxially growing fine grains of hexagonal close packed (HCP) Co alloys deposited thereon.
As the demand for high areal recording density increases, the requirements for high recording signal, low media noise and narrow transitions become increasingly difficult to simultaneously satisfy, thereby imposing great demands on film structure design and fabrication techniques. Thus, efforts have been made to explore new types of magnetic materials, explore new underlayer materials, design new multi-layer thin film structures and manipulate various thin film deposition parameters in attempting to improve magnetic properties and information read/write processes. However, there remains a need for increasingly high areal recording density magnetic recording media exhibiting a Hr, high SNR, and reduced recording signal modulation.